Defects in metallic structures may reduce the service life of a machine part, structural component, pipe, turbine, or other component. Defects may include residual stresses, delamination, porosity, cracks, microcracks or other features in a material due to a manufacturing process such as heat treatment, cooling, annealing, forming, impurities, or welding. Any of these defects may be sites for initiation or propagation of cracks, for example. The cracks may lead to weakening and eventual failure of the material.
Methods for nondestructive testing of materials have been developed. The method of testing the welded structure or other machine part depends on the application that the part will be used. A weld for structural component may not be required to be as high a welding standard as a weld for pipe containing a hazardous component. Therefore, appropriate testing methods can assure a weld meets applicable standards. Currently, there are a variety of nondestructive testing methods including visual, dye penetrant, magnetic particle, ultrasonic and X-ray methods.
Visual Inspection
Visual inspection is often the most cost-effective method and is conducted during and after welding, but is the least reliable method. Visual inspection requires little equipment and involves prewelding visual inspection of the materials, confirmation of the correct welding equipment and material, and a visual inspection after each welding pass. During fabrication, visual examination of a weld bead and the end crater may reveal problems such as cracks, inadequate penetration, and gas or slag inclusions. Among the weld defects that can be recognized visually are cracking, surface slag in inclusions, surface porosity and undercut.
Visual inspection can only locate defects in the weld surface. Specifications or applicable codes may require that welds be tested more thoroughly than a mere visual inspection.
X-ray Inspection
X-ray is one of the most important, versatile and widely accepted of all the nondestructive examination methods. X-ray may be used to determine the internal soundness of welds. X-ray testing is based on the ability of X-rays and gamma rays to pass through metal and other solid materials and produce photographic images of the transmitted radiant energy. All materials will absorb known amounts of this radiant energy and, therefore, X-rays and gamma rays can be used to show discontinuities and inclusions within the solid material.
X-rays or gamma rays are directed at a section of weld and only a portion of the rays pass entirely through the metal. Variations in the how the rays pass through and are recorded on a radiograph is indicative of variations within the metal. A material should absorb a certain amount of radiation, however, where the radiographs show less absorption, there may be a thin section or a void in the weld. The reliability and interpretive value of radiographs is a function of their sharpness and contrast.
Radiographs may be difficult to analyze. Filmhandling marks and streaks, fog and spots caused by developing errors may make it difficult to identify defects. Such film artifacts may mask weld discontinuities. Further, since surface defects show up on the film and may disguise defects and the angle of exposure also influences the image in the radiograph, it is difficult or impossible to evaluate fillet welds by this method.
Radiographic equipment produces radiation that can be harmful to body tissue in excessive amounts, so all safety precautions should be followed closely. All instructions should be followed carefully to achieve satisfactory results. Only personnel who are trained in radiation safety and qualified as industrial radiographers should be permitted to do radiographic testing.
Magnetic Particle Inspection
Magnetic particle inspection is a method of locating and defining discontinuities in magnetic materials. Magnetic particle inspection is excellent for detecting surface defects in welds, including surface cracks of all sizes in both the weld and adjacent base metal, subsurface cracks, incomplete fusion, undercut and inadequate penetration in the weld, as well as defects on the repaired edges of the base metal. In this method, probes are usually placed on each side of the area to be inspected, and a high amperage is passed through the workplace between them. A magnetic flux is produced at right angles to the flow of current. When the magnetic flux encounters a discontinuity, such as a longitudinal crack, they are diverted and leak through the surface, creating magnetic poles or points of attraction. A magnetic powder dusted onto the surface will cling to the leakage area more tenaciously than elsewhere, forming an indication of the discontinuity.
Although much simpler to use than radiographic inspection, the magnetic particle method is limited to use with ferromagnetic materials and cannot be used with austenitic steels. This method is best with elongated forms, such as cracks, and is limited to surface flaws and some subsurface flaws, mostly on thinner materials.
Dye Penetrant Inspection
Surface cracks and pinholes that are not visible to the naked eye can be located by dye penetrant inspection. It is widely used to locate leaks in welds and can be applied with austenitic steels and nonferrous materials where magnetic particle inspection would be useless.
Dye penetrant inspection is often referred to as an extension of the visual inspection method. In this method, a dye with good penetrating qualities is applied to the surface of the part to be examined. Capillary action draws the dye into the surface openings, and the excess present on the surface is then removed. A second developer is used to draw the dye from the cracks or pores to the surface. The presence of the dye indicates surface cracks or pores. The part to be inspected must be clean and dry, because any foreign matter could close the cracks or pinholes and exclude the dye. Dyes can be applied by dipping, spraying or brushing, but sufficient time must be allowed for the dye to be fully absorbed into the discontinuities. This may take an hour or more in very exacting work.
Ultrasonic Inspection
Ultrasonic Inspection is a method of detecting discontinuities by directing a high-frequency sound beam through the base plate and weld on a predictable path. When the sound wave encounters a defect in the material continuity, some of the sound is reflected back. The reflected waves are collected by the instrument, amplified and displayed as a vertical trace.
Both surface and subsurface defects in metals can be detected, located and measured by ultrasonic inspection, including flaws too small to be detected by other methods. The ultrasonic unit contains a crystal of quartz or other piezoelectric material encapsulated in a transducer or probe. When a voltage is applied, the crystal vibrates rapidly. As an ultrasonic transducer is held against the metal to be inspected, it imparts mechanical vibrations of the same frequency as the crystal through a coupler material into the base metal and weld. These vibrational waves are propagated through the material until they reach a discontinuity or change in density. At these points, some of the vibrational energy is reflected back. As the current that causes the vibration is shut off and on at 60-1000 times per second, the quartz crystal intermittently acts as a receiver to pick up the reflected vibrations. These cause pressure on the crystal and generate an electrical current. Fed to a video screen, this current produces vertical deflections on the horizontal base line. The resulting pattern on the face of the tube represents the reflected signal and the discontinuity. Compact portable ultrasonic equipment is available for field inspection and is commonly used on bridge and structural work.
Ultrasonic testing is less suitable than other NDE methods for determining porosity in welds, because round gas pores respond to ultrasonic tests as a series of single-point reflectors. This results in low-amplitude responses that are easily confused with “base line noise” inherent with testing parameters. Ultrasonic examination requires expert interpretation from highly skilled and extensively trained personnel.
In the art, the excitation and registration of the acoustic waves is performed by piezoelectric devices with inclined introduction emission. These systems do not allow for analysis to be carried out in a sufficiently wide band of ultrasonic frequencies and to ensure a sufficient locality of measurement of the stressed state of material and it is not possible to ensure both high accuracy and locality measurements needed in many measurement applications.
There exists a need for a nondestructive testing apparatus and method that is relatively inexpensive, accurate, and easy to use and easy to analyze the results.